Observations of High-frequency Energy from the 2021 M8.2 Chignik Earthquake on a Hydrophone Streamer

The M8.2 Chignik earthquake of July 28th, 2021 occurred along the Alaska-Aleutian subduction zone south of the Alaska Peninsula. At the time of the earthquake, the R/V Marcus G. Langseth was collecting seismic reflection data in the northern Pacific along the southern Queen Charlotte fault system just offshore of Haida Gwaii, British Columbia. The R/V Langseth was towing a 15-km-long, 1200-channel Sercel 3 Hz streamer which detected high-amplitude T phase arrivals from the earthquake. The arrival time of the T phase energy at the streamer hydrophones is consistent with the distance to the epicenter of the earthquake assuming a velocity of 1500 m/s; this suggests that T phase energy radiated directly from the earthquake epicenter and traveled through the SOFAR channel to the hydrophone streamer. Using the streamer orientation and an apparent velocity of the arrivals of ~1800 m/s, we calculated an azimuth that is also consistent with the earthquake epicenter location. The direct T phase was dominated by ~2.5 7 Hz energy, with peak power at ~5.5 Hz. In addition to the high-amplitude direct T phase arrivals from the M8.2 earthquake epicenter, T phase arrivals from multiple aftershocks were also recorded by the streamer system. T phase arrivals evident in several shot gathers provide information about the duration of T phase energy. Additionally, shot gathers collected prior to the direct T phase arrival show evidence for high-frequency earthquake energy generated from faster-traveling body waves. This recorded event provides a unique opportunity to investigate the sources, attenuation, frequency content, and other characteristics of high-frequency energy generated by a great subduction zone earthquake.